The invention relates to a mold for vertical electromagnetic continuous casting of rolling slabs, said mold having an inductor and partly covering this an electromagnetic screen such that the screen with respect to the inductor is arranged at the broad sides of the mold in a way that the electromagnetic turbulence created in the molten head of the ingot being cast is insignificant.
By the term rolling slabs is to be understood continuously cast ingots having board sides that represent the rolling surface and narrow sides that, together with the former, form an essentially rectangular cross-section.
In the process of electromagnetic continuous casting the heat in the ingot strand emerging from the mold is extracted by jetting the ingot surface with coolant immediately below the mold. During the start-up phase the coolant only strikes the dummy base. The resultant indirect extraction of heat leads to slow solidification of the molten metal and hence to a flat ingot foot. With progressive lowering of the dummy base the coolant strikes the surface of the ingot directly, which produces a sudden increase in the rate of heat extraction from the ingot. The thermal stresses resulting from this thermal shock are greater than the yield stress of the ingot and lead to permanent deformation in the form of a convex curvature of the ingot foot.
In order to keep the ingot foot as flat as possible, one can for example reduce the cooling intensity at least during the start-up phase. In one known process use is made of a coolant that has gas dissolved in it, this causing an insulating film to form on the ingot surface and with that a reduction in heat extraction.
A basic advantage of electromagnetic casting as compared with conventional continuous casting is the uniform ingot surface that forms, free of cold shuts, bleeding, surface segregation and other near surface defects, thus in most cases making it unnecessary to scalp the ingot surface.
The electromagnetic field created by the inductor induces circulation of the melt in the molten ingot head. This electromagnetic turbulence can, among other things, lead to the oxide skin being torn away and consequently to a worsening of the solidification conditions and the melt quality in the region of the solidifying ingot surface. This expresses itself for example as an accumulation of oxide inclusions, in longitudinal folds and as surface defects that do not appear until the material has been processed further e.g. slivers, looper lines and the like. Particularly sensitive are of course alloys with a strong tendency to oxidize such as aluminum magnesium alloys having a magnesium content of 4% or more. Problems can also arise during the casting of special high grade materials that may require the ingot to be scalped; as a result it may not be possible to exploit to the full the general advantages of electromagnetic casting.
A known method of minimizing the electromagnetic turbulence is to increase the covering of the inductor by means of the electromagnetic screen. On increasing the covering, however, the electromagnetic force directed at the center of the ingot is reduced. As a result problems can arise during start-up in that, due to the reduced electromagnetic force and the curvature of the ingot foot, molten metal can run out at the narrow side of the ingot, making it neseccary to halt the casting process.
In view of the above it is an object of the present invention to develop an electromagnetic mold of the kind mentioned at the start for continuous casting such that metal run-out from the ingot during the start-up phase can be prevented, this at the expense of an insignificant amount of melt turbulence in the molten head of the ingot being cast.